Get grub and gnulib:
$ git clone git://git.sv.gnu.org/grub.git
$ git clone git://git.sv.gnu.org/gnulib.git

Configure Grub:
$ cd grub
$ ./bootstrap --no-git --gnulib-srcdir=../gnulib                                                                                              
$ mkdir build
$ cd build
$ ../configure --target=x86_64 --disable-efiemu --with-platform=coreboot

Build Grub:
$ make -j4
$ make default_payload.elf

Get coreboot:
$ git clone https://review.coreboot.org/coreboot.git

Build Coreboot:
$ cd coreboot
$ make menuconfig
$ make

Exchange Payload:
$ build/util/cbfstool/cbfstool build/coreboot.rom remove -n fallback/payload -r COREBOOT
$ build/util/cbfstool/cbfstool build/coreboot.rom add-payload -f ../grub/build/default_payload.elf -n fallback/payload -r COREBOOT
$ cd ..

Get minmal Linux kernel for testing purpose:
$ wget https://debug.openadk.org/qemu-x86_64.img

Start Qemu:
$ qemu-system-x86_64 -bios coreboot/build/coreboot.rom -drive if=none,id=stick,format=raw,file=qemu-x86_64.img \
 -device nec-usb-xhci,id=xhci -device usb-storage,bus=xhci.0,drive=stick -serial stdio

Grub console:
grub> set root=(usb0)
grub> linux (usb0)/kernel
grub> boot

If you want to use an USB stick connected to your host you need to start Qemu as root
like this. Find out your USB product and vendor id via lsusb:

$ sudo qemu-system-x86_64 -bios coreboot.rom -drive if=none,id=stick,format=raw,file=qemu-x86_64.img  -device nec-usb-xhci,id=xhci -device usb-storage,bus=xhci.0,drive=stick -serial stdio -device usb-host,bus=xhci.0,vendorid=0x058f,productid=0x6387